1. Field of the Invention
The present invention relates to a nitride semiconductor device. More particularly, the present invention relates to a nitride semiconductor device that uses a nitride semiconductor substrate of which at least the surface is formed of a nitride semiconductor, and to a fabrication method of such a nitride semiconductor device.
2. Description of Related Art
Nitride semiconductors such as GaN, AlGaN, GaInN, and AlGaInN are characterized by having larger band gaps Eg than AlGaInAs- or AlGaInP-based semiconductors and by being direct-transition semiconductor materials. For these reasons, nitride semiconductors have been receiving much attention as materials for producing semiconductor light-emitting devices such as semiconductor lasers that can emit light at short wavelengths from ultraviolet to green regions of spectrum and light-emitting diodes that cover a wide range of light emission wavelengths from ultraviolet to red regions of spectrum. Thus, nitride semiconductors are expected to find wide application in high-density optical disc drives, full-color displays, and other appliances, and further in environmental, medical, and other fields.
Moreover, nitride semiconductors have higher thermal conductivity than GaAs-based or other semiconductors, and are thus expected to find application in devices that operate at high temperatures and high outputs. Furthermore, nitride semiconductors do not require any material such as arsenic (As) as used in AlGaAs-based semiconductors or cadmium (Cd) as used in ZnCdSSe-based semiconductors, nor do they require any source material or the like therefor such as arsine (AsH3), and are thus expected as compound semiconductor materials that are environmentally benign.
Conventionally, however, the fabrication of, among different types of nitride semiconductor devices, nitride semiconductor laser devices suffers from extremely low yields; that is, relative to the total number of nitride semiconductor laser devices fabricated on a single wafer, the number of non-defective ones is extremely low. One reason for low yields is believed to be development of cracks. The cause for cracks may lie in a substrate itself or in the process of laying on a substrate a nitride semiconductor multiple-layer film composed of a plurality of nitride semiconductor layers (nitride semiconductor thin films) laid on one another.
It is intrinsically preferable to form a nitride semiconductor multiple-layer film, such as one formed of GaN, by growing it on a GaN substrate, because doing so helps produce a nitride semiconductor multiple-layer film with good crystallinity and few defects. Up to date, however, no high-quality GaN single-crystal substrate has been developed that exhibits good lattice match with GaN. For this reason, a SiC substrate is often used instead because of the comparatively small difference in lattice constant. Disadvantageously, however, SiC substrates are expensive, are difficult to produce in large diameters, and develop tensile strain. As a result, SiC substrates are prone to develop cracks. Moreover, the substrate of a nitride semiconductor is required to be formed of a material that withstands a high growth temperature of about 1000° C. and that resists discoloration and corrosion in an atmosphere of ammonia gas used as a source material.
Out of the above considerations, a sapphire substrate is typically used as a substrate on which to lay a nitride semiconductor multiple-layer film. Sapphire substrates, however, cause large lattice mismatch (about 13%). For this reason, on a sapphire substrate, first a buffer layer formed of GaN or AlN is formed by low-temperature growth, and then, on this buffer layer, a nitride semiconductor multiple-layer film is grown. Even then, it is difficult to completely eliminate strain, resulting in development of cracks depending on conditions such as the film composition and film thickness.
The cause for such cracks may lie elsewhere than in the substrate, as will be described below. When a nitride semiconductor laser device is fabricated, a nitride semiconductor multiple-layer film is laid on a substrate, and the nitride semiconductor multiple-layer film is composed of different types of film, such as GaN, AlGaN, and InGaN. Here, the individual films of which the nitride semiconductor multiple-layer film is composed have different lattice constants, and thus exhibit lattice mismatch, causing development of cracks. As a countermeasure against this, there has been proposed a method for reducing cracks according to which a processed substrate is used so that, after a nitride semiconductor multiple-layer film is grown thereon, the surface of the nitride semiconductor multiple-layer film is not flat but has concave areas formed thereon (see Japanese Patent Application Laid-open No. 2002-246698). By using, for example, the method disclosed in Japanese Patent Application Laid-open No. 2002-246698, it is possible to reduce cracks that develop as a result of lattice mismatch among the individual films of which a nitride semiconductor multiple-layer film formed on a substrate is composed. Disadvantageously, however, with the method disclosed in Japanese Patent Application Laid-open No. 2002-246698, the concave areas formed on the surface of the nitride semiconductor multiple-layer film degrade the flatness thereof.
As a countermeasure against such degraded flatness on the surface of a nitride semiconductor multiple-layer film, the inventors of the present invention have developed a method according to which trenches, in the form of one to several stripe-shaped grooves per nitride semiconductor laser device, and ridge portions, each located between two adjacent trenches and having a width of about 100 μm to 1 000 μm, are formed on a nitride semiconductor substrate and then, on this nitride semiconductor substrate, a nitride semiconductor multiple-layer film is laid. With this method, it is possible to prevent cracks, and simultaneously to obtain reasonably improved surface flatness on the surface of the ridge portions.
When a nitride semiconductor laser device is fabricated by the above-described method developed by the inventors of the present invention, a nitride semiconductor multiple-layer film is structured, for example, as shown in FIG. 20.
Specifically, the nitride semiconductor multiple-layer film 4 formed on the surface of a processed substrate 6 (see FIG. 19) formed of etched n-type GaN or the like has, for example, the following layers laid one after another in the order in which they are named on the surface of the processed substrate 6: an n-type GaN layer 200 having a layer thickness of 0.2 μm; an n-type Al0.05Ga0.95N first clad layer 201 having a layer thickness of 0.75 μm; an n-type Al0.05Ga0.92N second clad layer 202 having a layer thickness of 0.1 μm; an n-type Al0.05Ga0.95N third clad layer 203 having a layer thickness of 1.5 μm; an n-type GaN guide layer 204 having a layer thickness of 0.02 μm; an active layer 205 composed of three InGaN well layers each having a layer thickness of 4 nm and four GaN barrier layers each having a layer thickness of 8 nm; a p-type Al0.3Ga0.7N evaporation prevention layer 206 having a layer thickness of 20 nm; a p-type GaN guide layer 207 having a layer thickness of 0.02 μm; a p-type Al0.05Ga0.95N clad layer 208 having a layer thickness of 0.5 μm; and a p-type GaN contact layer 209 having a layer thickness of 0.1 μm. The active layer 205 has the following layers formed one after another in the order in which they are named: a barrier layer, a well layer, a barrier layer, a well layer, a barrier layer, a well layer, and a barrier layer. In the following description, the term “p-layers” is used whenever necessary to refer to the nitride semiconductor layer composed of the layers doped with Mg laid on one another, namely the p-type Al0.3Ga0.7N evaporation prevention layer 206, the p-type GaN guide layer 207, the p-type Al0.05Ga0.95N clad layer 208, and the p-type GaN contact layer 209.
Thus, the nitride semiconductor multiple-layer film 4 is laid, by MOCVD, on the surface of the processed substrate 6, which has previously been processed. In this way, a nitride semiconductor wafer having concave areas on the surface of the nitride semiconductor multiple-layer film 4 as shown in FIG. 19 is produced. In FIG. 19, the plane directions are indicated together.
Used as the processed substrate 6 shown in FIG. 19 is an n-type GaN substrate that has trenches 2 and ridges 1, both in the shape of stripes, formed thereon in the <1-100> direction by a dry etching technique such as RIE (reactive ion etching). The trenches are formed with a width of 5 μm, with a depth of 5 μm, and with a period of 350 μm between two adjacent trenches. On the processed substrate 6 so etched, the nitride semiconductor multiple-layer film 4 having a layered structure as shown in FIG. 20 is formed by a growth technique such as MOCVD.
When nitride semiconductor laser devices were actually fabricated by the above-described method developed by the inventors of the present invention, by using an n-type GaN substrate as the processed substrate 6 and then epitaxially growing the nitride semiconductor multiple-layer film 4 on the n-type GaN substrate by MOCVD or the like, it was found that the method was effective in reducing cracks but not in notably increasing yields. Specifically, by the above-described method, a plurality of nitride semiconductor laser devices were fabricated, from which 100 nitride semiconductor laser devices were then randomly sampled and subjected to measurement of the FWHM (full width at half maximum) of their FFP in the horizontal and vertical directions. Here, nitride semiconductor laser devices whose actual FFP FWHM was within ±1 degree of the design value thereof were counted as non-defective ones. The result was that only 30 nitride semiconductor laser devices met the requirement for the FWHM of their FFP, indicating a very low yield.
This is because the surface of the nitride semiconductor multiple-layer film 4 formed was not sufficiently flat. With insufficient surface flatness, the layer thicknesses of the individual layers vary within the nitride semiconductor multiple-layer film 4, causing variations in characteristics among individual nitride semiconductor laser devices. This reduces the number of devices that have characteristics within the required ranges. Accordingly, to increase yields, it is necessary not only to reduce development of cracks, but also to make the layer thicknesses more uniform and the film surface more flat.
It was also found that, when an electrode pad is formed on a surface that is poorly flat because of concave areas, current leaks via those concave areas, making it impossible to obtain a normal current-voltage (V-V) characteristic in lasers. Basically, an insulating film such as SiO2 is formed on concave areas, and electrode pads are formed further on top. Here, if the surface has non-flat regions such as concave areas, the insulating film formed thereon is not formed uniformly. When the insulating film was analyzed, it was confirmed that it had many regions where small cracks and pits had developed and where the insulating film was extremely thin. And it was found that this non-uniform insulating film was the cause for current leakage.
Moreover, surface flatness was measured within the wafer surface of the nitride semiconductor wafer produced as shown in FIGS. 19 and 20. The result of the measurement of surface flatness in the <1-100> direction is shown in FIG. 21. The measurement was performed under the following conditions: measurement length, 600 μm; measurement duration, 3 s; probe pressure, 30 mg; and horizontal resolution 1 μm per sample. Within the 600 μm wide region so measured, the level difference between the highest and lowest parts of the surface was, as will be understood from the graph in FIG. 21, 30 nm. In this measurement, the nitride semiconductor wafer is assumed to have an off-angle of 0.02° or less.
This difference in flatness results from the fact that, as shown in FIG. 19B, the film thicknesses of the individual layers of the nitride semiconductor multiple-layer film 4 laid on the surface of the processed substrate 6 vary from place to place within the surface of the wafer. As a result, the characteristics of nitride semiconductor laser devices vary depending on where on the surface of the wafer they are fabricated, and the Mg-doped p-layer thickness (corresponding to the sum of the thicknesses of the p-layers laid on one another as the p-type Al0.3Ga0.7N evaporation prevention layer 206 to the p-type GaN contact layer 209 shown in FIG. 20), which greatly affects the characteristics of nitride semiconductor laser devices, greatly varies from place to place within the surface of the substrate.
When a ridge structure as a current constriction structure is formed, ridge portions are left in the shape of 2 μm wide stripes and other portions are etched away by a dry etching technique using an ICP (inductively coupled plasma) machine. Thus, if the p-layer thickness before etching varies from place to place within the surface of the wafer, the remaining p-layer film thickness, that is, the p-layer thickness that remains after etching and that therefore most greatly affects the characteristics of nitride semiconductor laser devices, also greatly varies from place to place within the surface of the wafer. As a result, not only do the layer thicknesses vary among nitride semiconductor laser devices, but also, even within a single nitride semiconductor laser device, the remaining p-layer film thickness is almost zero in some parts thereof while being considerably large in other parts thereof. When the remaining p-layer film thickness varies in this way, it affects the lifetime of nitride semiconductor laser devices and also, as described above, the characteristics thereof such as the FFP (far-field pattern).
The reason that a large layer thickness distribution exists within the wafer surface as described above is believed to be that the growth speed of the film grown epitaxially on ridge-shaped portions of the processed substrate including the nitride semiconductor substrate varies under the influence of trenches, resulting in degraded uniformity within the wafer surface.
Specifically, as shown in FIG. 22, on a processed substrate 6 having trenches 2 formed thereon, when epitaxial growth is started, at an initial stage of growth, as shown in FIG. 22A, trench growth portions 222 formed by the nitride semiconductor thin films grown on floor portions 224 and side portions 226 of the trenches 2 fill only part of the trenches 2. Meanwhile, top growth portions 221 formed by the nitride semiconductor thin films grown on the surface of top portions 223 of ridges 1 grows while keeping the surface of the nitride semiconductor thin films flat.
The epitaxial growth of the nitride semiconductor thin films progresses from the above-described state shown in FIG. 22A to the state shown in FIG. 22B. In this state, the trench growth portions 222 formed by the nitride semiconductor thin films grown on the floor portions 224 and side portions 226 of the trenches 2 almost completely fill the trenches 2, and become linked, via growth portions 225, to the top growth portions 221 formed by the nitride semiconductor thin films grown on the surface of the top portions 223 of the ridges 1. In this state, the atoms or molecules (such as Ga atoms) that have deposited as a source material on the surface of the nitride semiconductor thin films grown on the top portions 223 of the ridges 1 undergo migration or the like under the influence of heat energy to move into the growth portions 225 or the trench growth portions 222. This movement of atoms or molecules resulting from migration thereof occurs non-uniformly within the wafer surface, and the movement distance varies from place to place within the wafer surface. As a result, as shown in FIG. 22B, the flatness of the surface of the top growth portions 221 is degraded.
The flatness of the nitride semiconductor thin films is degraded also in the <1-100> direction under the influence of non-uniformity of the nitride semiconductor substrate itself, such as the distribution of the off-angle within the wafer surface and the distribution of the substrate curvature within the wafer surface, or non-uniformity of the epitaxial growth speed within the substrate surface, or non-uniformity of the trench forming process within the substrate surface. Specifically, the time required to fill the trenches 2 varies depending on the <1-100> direction; thus, where they are filled earlier, the atoms or molecules of a source material of the nitride semiconductor thin films migrate or otherwise move from the top growth portions 221 of the ridges 1 into the growth portions 225 or the trench growth portions 222. Where they have moved away, it takes more time to form the nitride semiconductor thin films, with the result that the nitride semiconductor thin films formed in the trenches 2 have larger thicknesses. By contrast, where the trenches 2 are filled later, the atoms or molecules of a source material of the nitride semiconductor thin films do not migrate or otherwise move from the top growth portions 221 of the ridges 1 into the trenches 2; even if they do, it takes less time to form the nitride semiconductor thin films. Thus, the nitride semiconductor thin films have smaller film thicknesses in these trenches 2 than where the trenches 2 are filled earlier.
In a state where the growth speed depends on the feed rate, that is, in a state where the growth speed of the nitride semiconductor thin films is controlled by the flux of the like of atoms or molecules fed to the wafer surface, when the atoms or molecules of a source material of the nitride semiconductor thin films migrate or otherwise move into the trenches 2, since the flux of the atoms or molecules of the source material fed to the entire wafer surface is constant, the film thicknesses in the top growth portions 221, where the nitride semiconductor thin films grow on the top portions 223 of the ridges 1, are smaller. By contrast, in a case where the atoms or molecules of a source material of the nitride semiconductor thin films do not migrate or otherwise move into the trenches 2, the film thicknesses in the top growth portions 221, where the nitride semiconductor thin films grow on the top portions 223 of the ridges 1, are larger.
Consequently, the layer thicknesses in the top growth portions 221 on the top portions 223 of the ridges 1 vary within the wafer surface, with the result that the flatness of the surface of the nitride semiconductor thin films is degraded. Thus, to obtain better surface flatness, it is necessary to hinder the atoms or molecules of a source material of the nitride semiconductor thin films from migrating or otherwise moving from the top growth portion 221 on the ridges 1 into the growth portions 225 or the trench growth portions 22 and thereby hinder them from forming nitride semiconductor thin films there.
Another way to obtain better flatness is to make the atoms or molecules of a source material of the nitride semiconductor thin films, when they migrate or otherwise move from the top growth portions 221 on the ridges 1 into the trench growth portions, move uniformly all over the wafer.